The present invention relates to polymer nanofibres that are swellable and insoluble in an essentially aqueous effluent and use thereof in the treatment of these effluents.
The pollution of water by heavy metals has been an increasing environmental problem over the last few decades, requiring immediate and urgent action.
Heavy metals, such as Zn, Co, Fe, Cr and Cu, are widely used in the surface treatment industry, in connection with the automotive sector or microelectronics, for the implementation of electroplating, electroless or electrogalvanizing processes. Despite technical advances, the liquid effluents discharged by these industries inevitably always contain traces of heavy metals, mainly in the form of salts, which are dangerous to public health and the environment and require treatment before being discharged into the environment.
The European standards regulating the levels of the different elements discharged are now becoming increasingly strict. In the case of heavy metals such as copper, the discharge authorization threshold for the standard effluents has been reduced from 2 mg/L in 1985 to less than 0.5 mg/L since 1998, when the daily discharge quantity exceeded 5 g/day.
The nuclear industry also needs to address the issue of the treatment of liquid effluents contaminated with radioactive elements, which can be produced during decontamination operations or in the event of accidents. For example, the Fukushima disaster made vast quantities of seawater radioactive due to the presence of radioactive caesium. This resulted in thousands of cubic metres of seawater having to be treated in order to remove traces of radioactive caesium. The difficulty is to capture the caesium atoms present in very low concentrations, from 10−7 to 10−12 mol·L−1, among sodium atoms, which are at least 1000 times more concentrated.
In the case of the rare earths, in addition to concern for public health and the environment, the recovery and separation of these elements from an effluent is also of considerable economic significance. In fact, the rare earths are used in significant quantities in high-technology products and, on account of their strategic nature and supply difficulties, their recycling is becoming very attractive.
Consequently, the treatment of industrial liquid effluents is becoming a major problem in our society. Currently in industry, in particular in the surface treatment, electronic, or nuclear industries, there is a specific demand for novel processes that are cleaner, inexpensive and able to come down to a performance threshold up to ten times lower than the current standards.
Most conventional methods for the treatment of effluents comprise a primary treatment followed by a secondary treatment. The primary treatment implemented by coagulation and/or precipitation makes it possible to remove the different solid pollutants. If this treatment also makes it possible to reduce the metals present in an effluent by 90% to 95%, it does not always make it possible to meet the requirements of the standards. It is necessary to use a secondary treatment in order to further reduce the heavy metals content.
The following conventional methods that can be used for a secondary treatment may be mentioned as examples: ion-exchange resins, membrane filtration, liquid-liquid extraction. Some of these are fairly effective, but unfortunately have inherent limits such as rigorous operating conditions and the production of secondary contaminants.
Ion-exchange resins are widely used for treating effluents containing heavy metals and have numerous advantages: their treatment capacity, a certain metal capture selectivity and their rapid kinetics compared with other, for example membrane, techniques. Most resins are constituted by very small synthetic polymer beads (50-500 μm) between which the effluent passes in order to be decontaminated. The effectiveness of these resins is however limited due to their design. In fact, the vast majority of resins reach only 50% of their exchange capacity due to their manufacture which requires chemical functionalizations that have not taken place at all the potentially accessible sites. On the other hand, the ion-exchange resins are especially effective at their periphery where the exchange can take place rapidly. As diffusion of the liquid in solid phase is very slow, diffusion channels are designed in the manufacture of these resins in order to improve the penetration of the effluent into the core of the resin beads. Nevertheless the diffusion of the liquid deep in the resins is still slowed and this contributes to reducing the useful capacity of the resins. It would be possible to compensate for this problem by drastically reducing the diameter of the beads in order to reduce the surface/volume ratio. However, problems of pressure loss (clogging) and risks presented by their powdery nature will then become apparent.
Another major drawback of this technique is the need to use large quantities of concentrated acids and bases in order to carry out the regeneration operations. It is acknowledged that the regeneration of a kilogram of ion-exchange resins can require the use of more than 50 litres of acids and/or of bases which become secondary effluents. It is important to minimize these secondary effluents.
Another technology developed in the last twenty years consists of using membrane techniques, such as microfiltration, nanofiltration, ultrafiltration and reverse osmosis. However, at present, these techniques are rarely used in the separation of metals, as the membranes can be mechanically fragile and do not have properties of selectivity vis-à-vis metals. Moreover, these techniques consume a great deal of energy.
More recently, natural fibres, such as fibres originating from cotton, fungi, cacti, and waste from food-producing agriculture have been used for the filtration and extraction of metals. The manufacture of these fibres is very cost-effective, but their performance and especially their selectivity remain very modest. For example, the performance of a membrane obtained from a mixture of natural cotton and silk fibres in order to capture copper is only 2.88 mg/g (Ki et al., 2007, Membrane Sci., 302, 20).
The latest technological development consists of using nanofibres, which have micrometric or submicronic diameters and are manufactured either by electrospinning of a polymer solution under high voltage or by application of a centrifugal force (centrifugal spinning) to a polymer solution. However, the industrial use of these nanofibres, often in membrane form, at present remains within the field of tissue engineering or as super-absorbent materials, or also for carrying out mechanical filtration.
To date, several scientific publications have discussed the application of membranes of nanofibres obtained from different polymers for capturing metal ions in solution. However, the performance of these membranes is not satisfactory and does not allow large-scale productivity.
Ignatova et al. (Macromol. Rapid Commun. 2008, 29, 1871-1876) describe the use of nanofibres of polystyrene associated with calixarenes for capturing nickel. No capture capacity is mentioned in this article. The polystyrene is recognized as being very hydrophobic and not very swellable in water.
Numerous publications are also to be found concerning fibres of polyacrylonitrile or derivatives thereof. (F. Huang, Materials 2013, 6, 969-980). Polyacrylonitrile is also recognized as being not very hydrophilic and therefore not very swellable in water.
Wang et al. (J. Membr. Sci. 2011, 379 191-199) have analysed a membrane obtained by electrospinning from a solution of polyvinyl alcohol (PVA) and polyethylenimine (PEI). Its capacity for capturing copper is 67.16 mg/g. However, the regeneration of this membrane requires either very strong acid or basic solutions or a powerful and toxic organic complexing agent, such as EDTA. Moreover, these polymers are not sufficiently hydrophilic for the treatment of substantially aqueous industrial effluents.
Xiao et al. (J. Appl. Polym. Sci., 2010, 116, 2409) have described a membrane obtained by electrospinning from a solution of polyacrylic acid (PAA) and polyvinyl alcohol (PVA). Its copper capture performance is approximately 10 mg/g and is therefore still very low. The incorporation of PVA into the initial polymers reduces the hydrophilic properties of the nanofibres, limiting the solubilization of the PAA.
Li and Hsieh (Polymer 2005, 46, 5133) have described the possibility of cross-linking a pure PAA nanofibre with beta-cyclodextrins. The authors chose to react the alcohol functions of the cyclodextrins with the anhydride functions of the PAA which are formed by annealing at 140° C. However, the permeability of this material is not sufficient, as the cyclodextrin content of approximately 30% remains high. Moreover, the tests for this material were carried out in an acid aqueous medium of pH 2-7 and over a maximum period of 24 h. This range of pH has no technical benefit in the field of retreatment of metals with PAA, as the latter acquires its complexing form in basic media.
As an alternative to electrospinning, techniques based on centrifugal force (centrifugal spinning) have recently been developed, such as for example, the FORCESPINNING® technique (Sarkar et al., Materials Today, 2010, 13(11), p 12-14). However, to date nanofibres obtained by this technology have not been used in the separation of ions in solution.
The nanofibres described in these previous publications cannot be used on an industrial scale for extracting and separating the metals because, on the one hand, they are not sufficiently hydrophilic in a substantially aqueous effluent and do not have a satisfactory capacity or specificity for capturing the metals and, on the other hand, they additionally require a very long capture or regeneration time.
In response to this problem, there is very strong interest in developing nanofibres which have a very high water-permeability and which do not dissolve. In order to improve the properties of these nanofibres vis-à-vis metal targets that are difficulted to extract or interfered with by elements of little interest, these nanofibres will be able to contain complexing molecules having very specific capture properties.